For example, you might not serialize a document, but you would serialize the items that are part of the document. By calling MapObject, you allow those items, or subobjects, to reference the document. Also, serialized subitems can serialize their m_pDocument back pointer.

You can call MapObject when you store to and load from the CArchive object. MapObject adds the specified object to the internal data structures maintained by the CArchive object during serialization and deserialization, but unlike ReadObject and WriteObject, it does not call serialize on the object.